A variety of techniques have been used in determining the presence quantities of hydrocarbons (oil and gas) in earth formations. One such technique involves the use of NMR logging tools for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, spin-lattice relaxation time (generally referred to in the art as “T1”), and spin-spin relaxation time (generally referred to as “T2”) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A conventional “side-looking” NMR tool is sensitive to NMR excitation on one side of the tool and less sensitive to NMR excitation on the other side. The more sensitive side of the tool is typically pressed against the side wall of a borehole adjacent a formation, thereby providing minimum separation between the NMR tool's radio frequency (“RF”) field generating assembly and the formation volume of NMR investigation. The less sensitive side of the tool is thus exposed to the borehole, which gives rise to the issue of borehole signal whereby the NMR signal from the borehole tends to erroneously contribute to the received NMR signal (i.e., “total signal” as referred to herein) from the volume of interest.
A variety of conventional techniques have been utilized to combat the issue of the borehole signal. In one example, a side looking NMR tool design includes a dipole RF antenna displaced toward the formation and a single permanent magnet, having a single magnetization direction, placed toward the back of the tool. However, for such a configuration, the signal from borehole will still be unacceptably large for larger boreholes. To solve this problem, other approaches have utilized a tool having an active RF spoiler design where a spoiler in the back, possessing a dipole moment opposite to that of the front antenna, spoils the borehole signal to an acceptable level. However, a spoiler implementation introduces a high level of complexity in terms of building and testing of the tool, as well as reliability issues.
Therefore, there is a need in the art to provide alternative, reliable and less complex NMR logging tools capable of eliminating and/or reducing the borehole signal to an acceptable amount.